Telephone system



Aug. 28, 1951 J. wlcKs ETAL TELEPHONE SYSTEM 15 Sheets-Sheet 1 Filed Jan. 29, 1949 INVENTORS John W|cks BY v Oho D. Grandsraff oom monaco@ J. wlcKs ET AL 2,566,010

TELEPHONE SYSTEM 15 Sheets-Sheet .2

Aug. z8, 1951 Filed Jan. 29, 1949 K 15 Svheeis-Sheet 3 J. wlcKS E T AL TELEPHONE 'SYSTEM Aug. 28, 1951 Filed Jan. 29, 1949 Aug.28,1951 l JMCKS'TAE v12,5663010 TELEPHONE SYSTEM Filed Jan. 29, 1949 i 15 sheets-sheet 4 c392`Q C393 REPEATER CIR. 40o

, TANDEM 22 TEST sE 50 ANS. sElzE f E v l INVENTORS 4 John Wicks a BY Ofho D GrohdsTuff Aug.2,8,1951 J.w|Ks Em. 2,566,010`

TELEPHONE SYSTEM Filed Jan'. 29, 1949 l5 Sheets-Sheet 5 TEST 53 REPEATER 400 SE I'ZURE INVENTORS l u John Wicks l F G 5 By Orho D. Grandstaff Atys.

Aug- 28, 1951 J. WICKS ET AL 2,566,010

' TELEPHONE SYSTEM n Filed Jan. 29, 1949 l5 Sheets-Sheet 6 REPEATER CIR 40o C6 PULSE CUT IN 684 l ausY L END A 675 ToNE yUNIT ses 6 PREP 60' 626 R670 686 A SUPY. PULSE INVENTORS F/G- 6 John Wicks BY Ofho D. Grondsfaff M, 05mm AHvs.

Aug. 28, 1951 J. WrCKS m.. 2,566,010

TELEPHONE SYSTEM Filed Jan:' 29, 1949 15 Sheets-Sheet 7 REPEATER CIR. 700

lVoice Current 733\1\! v R7 SEND 752ml:

PAD CONI IME PULSE Tarea John Wicks By Oho D. Grnndsaff M, QM

Aug. 2s, 1951 J. WICKS Em 2,566,010

TELEPHONE SYSTEM Filed Jan. 29, 1949 15 sheets-sheet AFIG. 8 l 88 RERING R8 John Wicks f BY Otho D. Grands'raff Aug. 2s, 1951 J wiCKg TAL' '2,566,010

TELEPHONE SYSTEM Filed Jan. 29, 1949 15 Sheets-Shes?. 9

94 5 9H SI2/L, 98

FINDER MoN.

. INVENTORS John Wicks F G. 9 By omo D. srandsmff A g I c c' t i I t Aug. 28, 1951 Filed Jan.

TELEPHONE SYSTEM l5 Sheets-Sheet l0 Aug. 28, 1951 J. wlc'Ks ET AL 2,566,010

TELEPHONE SYSTEM Filed Jan. 29, 1949 15 sheets-sheet 11 Aug. 2s, 1951 J. wlKs ETAL TELEPHONE SYSTEM 15 Sheets-Shea?l 12 Filed Jan. 29, 1949 INVENTORS John Wicks Ofho D.'Grandsnf.f mrk 06M QM Aug. 28, 1951 J. WlcKs ET Al. 2,566,010

n TELEPHONE SYSTEM I Filed Jan. 29, 1949 15 Sh'eets-Sheet 13 John Wicks By Oho D. Grandsiaff Atrys;

Aug. 28, 1951 J. wlcKs ET AL TELEPHONE SYSTEM 15 sheets-sheet' 14 Filed Jan. 29, 1949 BY Ofho D. Grandsraff M oam M Aug. 28, 1951 J. wlcKs ET AL TELEPHONE SYSTEM 15 Sheets-Sheet 15 COQ tz: m zo @im .s Een: JY M5012@ @Q zoom To Assoc. REPT. clR. JVENTORS John Wicks Oho D. Grondsaff Patented Aug. 28, 1951 2,566,010 TELEPHONE 'SYSTEM John Wicks, Biloxi, Miss.,

and Otho D. Grandsta, Oak Park, Ill., assignor's to Automatic Electric Laboratories, In poration of Delaware riginal application May c., Chicago, Ill.,A a cor- 22, 1947, serial No.

749,758. Divided and this application January 29, 1949, Serial No'. 73,482'

The present invention relates to telephone systems and more particularly to toll switching systems of the type disclosed in U. S. Patent No. 2,155,176, John Wicks, granted April 18, 1939. This application is a division of the copending application of John Wicks and Otho D. Grandstafi, Serial No. 749,758, led May 22, 1947, now Patent No. 2,517,516, granted August 1, 1950.

In the toll switching system disclosed in the Wicks patent, the toll lines inter-connecting the dierent exchanges are terminated by repeater circuits, each of which is provided with a tone unit individual thereto of the connection and arrangement disclosed in U. S. Patent No. 2,159,081, Otho D. Grandstai, granted May 23, 1939. Each of these tone units comprisesl both a transmitter and a receiver of unmodulal-ed audio frequency signals and fixed frequency modulated audio frequency carrier signals that are employed for the purposes of digit control, seizure control, ring control, supervisory control and release control.

In the present toll switching system, improved repeater circuits are provided, each of which has individually associated therewith a tone unit of the improved connection and arrangement disclosed in the copending application of Otho D. Grandstaf, Serial No. 738,194, led March 29, 1947, now Patent No. 2,482,478, granted September 20, 1949.

It is a general object of the present invention to provide in a toll switching system of the type disclosed in the Wicks patent, improved circuit and operating facilities in order to accommodate the handling with ease of heavier toll traffic by the operators at the various toll s-Witchboards in the diierent exchanges in the system.

Another object of the invention is to provide in a telephone system of the type noted, an improved impedance balancing network in the repeater circuits inter-connected by the toll line in order to prevent singing of the voice current repeaters included in the toll line.

Further features of the invention pertain to the particular arrangement of the circuit ele# ments of the system, whereby the above-outlined and additional operating features thereof are attained.

The general arrangement of the telephone system 4 Claims; (Cl. 179-1631) drawings in which Figures 1 and 2 taken together, when arranged inv end-to-end' relation with Fig. 1 on the left and Fig. 2 on the" right, constitute a schematic diagram of the area served byV a' telephone system embodying the present in; vention and illustrate the general arrangement oi the apparatus incorporated in several of the exchanges and offices thereof; Figs. 3 to 14, inelusive, taken together illustrate the details of certa-in of the apparatus incorporated in tvvo of the remotely disposed automatic exchanges in'- cluded in the telephone system, which apparatus has incorporated therein the features of the invention as briefly outlined above; Fig. 15 illustrates the' mode of combining Figs. 3 to 14, inclusive, to form a unie'd diagram; andV Fig. 16 illustrates the details of one of the tone units incorporated in the apparatus illustrated in Figs. 3 to 14, inclusive.

More particularly, Figs. 4 to 6, inclusive, illus; trate' the details of one of the repeater circuits incorporated in one of the automatic exchanges; Figs. 7 to 13, inclusive, illustrate the details of one of the repeater circuits incorporated in another oi the automatic exchanges; and Fig. 14 illustrates a fragmentary portion of one of the coders associated with the repeater circuit illus"-` trated in Figs. 7 to 13,V inclusive.-

Referring now more particularly to Figs?. 1 and 2, the telephone system there illustrated was one applied'to meet the specific requirements of the system of toll lines inter-connecting a group of cities in Brazil, South America, and including the cities of Sao Paulo' and Rio de Janeiro. Since these twocitiescomprise the principal cities of Brazil, each serves as a concentration point for toll traiiic; and as these two cities are within about two hundred miles of each other considerable toll traiiic passes therebetween. In the general areaA of Sao Paulo are a numberlof cities, including Santos, Campinas'. and Jundiahy; an'din the general area `of Rio dei Janeiro are a number of cities, including Campos'. Sao Paulo comprises an automatic exchange 30 con` s-tituting a toll center; and a' number ofv auto# matic oiiice's, including the automatic office 383,r each serving a group of subscriber substations: The automatic oce 380 comprises oiiice No.l 8 in' Sao Paulo andserv'es' a group of subscriber substations, including the subscriber subst-ation TI, as previously noted. The switching apparatus incorporated in the automatic exchange 30, con# stitfuti''g a toll center, as well as theiswitching apparatusf incorporated in the automatic oiceA 383,. or the well-known Strowger type. santos comprises an automatic exchange 360 serving a group of subscriber substations, including the subscriber substation T2, and automatic switching apparatus of the well known Strowger type. Campinas comprises an automatic exchange 310 serving a group of subcriber substations, including the subscriber substation T3, and automatic switching apparatus of the well known Strowger type. Jundiahy comprises a manual exchange 300 and serves a group of subscriber substations, not shown.

Rio de Janeiro comprises an automatic exchange 50 constituting both a toll center and an oice serving a group of subscriber substations, including the subscriber substation T4. Also, the automatic exchange 50 comprises automatic switching apparatus of the well known International Standard Electric 7-A type, utilizing revertive impulse control. Campos comprises a manual exchange 60 and services a group of subscriber substations, not shown.

The automatic exchange 30 is connected to the automatic exchange 50 by a group of twoway toll lines, including the toll line 102. The toll line 102 embodies a number of voice current repeaters 103 distributed therealong; and the opposite ends thereof are terminated in repeater circuits 400 and 100, respectively disposed in the automatic exchange 30 and in the automatic exchange 50. A tone unit 60| in the automatic exchange 30 is individually associated with the repeater circuit 400 and a tone unit 10| in the automatic exchange 50 is individually associated with the repeater circuit 100.

The automatic exchange 30 is connected to the automatic exchange 360 by a group of twoway toll lines, including the toll line 356. The toll line 356 embodies a number of Voice current repeaters 351 distributed therealong; and the opposite ends thereof are terminated in repeater circuits 355 and 365, respectively disposed in the automatic exchange 30 and in the automatic exchange 300. A tone unit 358 in the automatic exchange 30 is individually associated with the repeater circuit 355 and a tone unit 36B in the automatic exchange 360 is individually associated with the repeater circuit 365.

The automatic exchange 30 is connected to the automatic exchange 310 by a group of two- Way toll lines, including the toll line 35|. The toll line 35| embodies a number of voice current repeaters 352 distributed therealong; and the opposite ends thereof are terminated in repeater circuits 350 and 315, respectively disposed in the automatic exchange 30 and in the automatic exchange 310. A tone unit 353 in the automatic exchange 30 is individually associated with the repeater circuit 350 and a tone unit 318 in the automatic exchange 310 is individually associated with the repeater circuit 315. The automatic exchange 30 is connected to the manual exchange 300 by a group of twoway toll lines, including the toll line 302. One end of the toll line 302 terminates in a manual toll board 30| disposed in the manual exchange 300, and the other end of the toll line 302 terminates in a toll trunk circuit 3|0 disposed in the automatic exchange 30.

The automatic exchange 30 is connected to the automatic oiice 380 by a group of outgoing trunk lines, including the outgoing line 342, and by a group of incoming trunk lines, including the `incoming trunk line 306. The outgoing trunk line 342 is accessible to the automatic switching apparatus in the automatic exchange the automatic exchange 30.

30 and terminates in the switch train 305 disposed in the automatic office 380. One end of the incoming trunk line 306 terminates in a trunk circuit 388 in the automatic ofce 380 and is accessible via a trunk 381 to the switch train 385; and the other end of the incoming trunk line 306 terminates in a CLR trunk circuit 305 in In passing, it is noted that the switch train 385 in the automatic office 300 has access to and is accessible to the subscriber line 386 extending to the subscriber substation Tl.

The automatic exchange 50 is connected to the manual exchange 60 by way of a group of twoway toll lines, including the toll line |36I. One end of the toll line |36| terminates at a manual toll board |320 disposed in the manual exchange 60, and the other end of the toll line I36| terminates in a toll trunk circuit I3|1 disposed in the automatic exchange 50.

The automatic exchange 30 further comprises a Strowger dial toll board, the rst operator position circuit of which is indicated at 3|6; which iirst operator position circuit 3i5 has access by way of an associated cable 354 to a number of finder-selector links, including the 'finder-selector link SI5. The nder-selectorlink 3|5 comprises a rotary nder having access to trunks 304 and 301, respectively extending to the CLR trunk circuit 305 and to the toll trunk circuit 3|0, and a Strowger selector. individually associated with the repeater circuits 350, 400 and 355, respectively terminating the toll lines 35|, 102 and 355 n the automatic exchange 30, are trunks 326, 390 and 33| respectively terminating in incoming toll selectors 325, 320 and 330. Also, the automatic switching apparatus in the automatic exchange 30 comprises a rst group of outgoing toll selectors, including the outgoing tol-1 selector 335, a second group of outgoing toll selectors, including the outgoing toll selector 340, and a third group of outgoing toll selectors, including the outgoing toll selector 3&5. The selector of each of the finder-selector links 3|5, etc., and each of the incoming toll selectors 325, 320, 330, etc., has access to each of the outgoing toll selectors 335, 340, 345, etc., via respectively associated trunks 336, 33|, 346, etc. The outgoing toll selector 335, etc., has access to a trunk 331 extending to the toll trunk circuit 3|0 and, consequently, to the toll line 302 extending between the automatic exchange 30 and the manual exchange 300. The outgoing toll selector 340, etc., has access to the outgoing trunk line 342 extending to the switch train 385 in the automatic office 380. The outgoing toll selector 345, etc., has access to trunks 395, 395 and 391 respectively extending to the repeater circuits 350, 500 and 355; and, consequently, to the respective toll lines 35|, 102 and 356 extending between the manual exchange 300 and the respective automatic exchanges 310, 50 and 360.

The automatic exchange 350 further comprises a Strowger dial toll board indicated at 365, and provided with automatic switching apparatus and control equipment indicated at 361. The automatic switching apparatus 361' has access to and is accessible to the subscriber line extending to the subscriber substation T2. Also, the switching apparatus 361 is accessible to and has access to the repeater circuit 365 via connections 356; and, consequently, with respect to the toll line 356 extending between the automatic exchange 360 and the automatic exchange 30. The automatic exchange 310 furthercomprises a Strowgei' dial toll board indicated at-BTQ, and provided with automatic switching apparatus and control equipment indicated at 3W. The automatic switch apparatus 3E? has access to and is accessible to the subscriber line extending to the subscriber substation T3. paratus 3H has access to and is accessible to the repeater circuit SI5 via connections 376; and, consequently, with respect to the toll line extending between the automatic exchange 310 and the automatic exchange 30.

The automatic exchange further comprises a manual dial toll board of the well known Western Electric Company No. 3 type, the first inward operator position circuit of which is indicated at iill, the first outward operator position circuit of which is indicated at |340, and the rst transfer operator position circuit of which is indicated at |350. The rst inward operator position circuit |3|6 is provided with an associated group of cord circuits, one of which is indicated at |3| 0, and provided with an answer plug i3! and a call plug |3| 2. Further, the rst outward operator position circuit |340 and the first transfer operator position circuit |355 are provided with associated groups of cord circuits, not shown, each terminating in answer and call plugs. Also, associated with the manual dial toll board is a display panel |336 that is connected to the repeater circuit "F00, etc., in order to display supervisory signals from the repeater circuit IES, etc.

- Further the repeater circuit is provided with two trunks |370 and |380. The trunk I3??` is an incoming trunk and extends to the automatic switch train |300 and is accessible thereto; while the trunk i 38] is a two-way trunk and terminates in jacks J|3|6, J|3fl| and J|35|, respectively disposed at the rst inward operator position circuit |3|5, at the first outward operator position circuit |340, and at the first transfer operator position circuit |353, disposed at the manual dial toll board in the automatic exchange 5t. Also, the automatic switch train |300 has access to and is accessible to the subscriber line |30| extending tothe subscriber substation T4. Further, the automatic switch train i305 has access to a group of outgoing trunks, including the outgoing trunk |3262, and is accessible to a group of incoming trunks, including the incoming trunk |363. The outgoing trunk |382 is terminated in a CLR trunk circuit |3i'i2 and the incoming trunk |363 terminates in a toll trunk circuit |303. The toll trunk circuit |303, etc., the CLR trunk circuit |302, etc., andthe toll trunk circuit |3|l, etc., respectively terminate in jacks J|308, M309 and J|3i3 disposed at the rst inward operator position circuit SIS and inultipled to similar jacks, not shewn, disposed at the first outward operator position circuit |340 and at the rst transfer operatcr position circuit |350.

Further, a group of toll register senders, including the toll register sender |39I, and a group of local register senders, including the local register sender |332, are commonly associated with the automatic switch train |300 in the automatic exchange 5.0. Further, in the automatic exchange there is disposed a preselector |403 having access to a group of coders, including the coder |404, and-.to the repeater circuit ID3, etc. Specifically, the preselector |463 has access to the coder |404 via, a cable |620 and access to the repeater circuit |00 vi-a acable Mid.. Also, the automatic exchange 50 comprises a group of finder circuits, including the finder circuit Edili, and an associated group of register senders, including. the register Also, the switching apsender |402, as well as an associated allotter |405. The allottcr |405 is commonly associated with repeater circuit 100, etc., and with the nder circuit |40|, etc.; while the nder circuit |40l, etc., has access to the repeater circuit "|00, etc., via a cable |090, etc. Finally, the nder circuit |40| is individually associated with the register sender |402.

The apparatus incorpo-rated in the telephone system In the automatic oice 300 the switch train 385 is of the well known Strowger type, as previously noted, while the trunk circuit 38B- may be of any suitable type'. A

In the automatic exchange 360 the toll switchboardV 309 is of the well known Strowger dial type, as previouslynoted, and may be identical to the Strowger dial toll board provided in the automatic exchange 30. similarly, the automatic switch and controlv equipment 30'! disposed inthe automatic exchange 35.0" is of the well known Strowger type, as previously noted, andmay be identical to the corresponding automatic switch and control equipment disposed in the automatic exchange 30. Further, the repeater circuit 365 disposed in the automatic exchange 300 is identical to the repeater circuit 355 disposed in the automatic exchange 30; which, infturn, is identical tothe repeater circuit 400 described in detail hereinafter. Finally, the tone unit 368 disposed in the automatic exchange 350 is identical to the tone unit 358 disposed in the automatic exehange; which, in turn, is identical to the tone unit |6051 describedv in detail hereinafter.

In the automatic exchange 310 the toll switchboard 359 is of the well known Strowger dial type,

as previously noted, and may be identical to the Strowger dial toll board provided in the automatic exchange 33. Similarly, the automatic switch and control equipment 37T disposed in the automatic exchange 3?@ is of the well known Strowger type, as previously noted, and may be identical to 'the corresponding automatic switch and control. equipment disposed inthe automatic exchange 30. Further, the repeater circuit 315 disposed in the automatic exchange is ide-ntical to the repeater circuit 35d disposed' in the automatic exchange 30; which, in turn,is identiycal to the repeater circuit lili? described in detail hereinafter. Finally, the tone unit 3'i3 disposed in the automatic exchange 310 is identical to the tone unit 353 disposed in the automatic exchange 33; which, in turn, is identical to the tone unit |600 described in detail hereinafter.

In the automatic exchange 30, preferably the CLR trunk circuits 305, etc., are identical and are of the form illustrated in Fig. 3 of the previously mentioned Wicks patent; and the toll trunk circuits 3m, etc., are identical and are of the form illustrated in Fig. 4 of the previously mentioned Wicks patent. Also, in the automatic exchange 33 the various operator position circuits 3MB, etc., at the Strowger dial toll board therein are identical and are of the form illustrated in 5 of the previously mentioned Wicks patent. Similarly, the under-selector links 3| 5, etc., are identical and are preferably of the forni illustrated in Figs. 5 and 6 of the previously mentioned Wicks patent. Preferably, the incoming toll selectors 325., 320,330, etc., are identical and are oi' the form illustrated in Fig. dof the previorsly nientioned Wicks patent; the outgoing toll selectors 345, etc., are identical and are of the i'orni illustrated in Fig. 8 of the previously mentioned Wicks patent; the outgoing toll selectors 340, etc., are identical and are of the form illustrated in Fig. 9 of the previously mentioned Wicks patent; and the outgoing toll selectors 335, etc., are identical and are of the form illustrated in Fig. 10 of the previously mentioned Wicks patent. As previously noted, the repeater circuits 350, 355, etc., are identical to the repeater circuit 400 illustrated in Figs. 4 to 6, inclusive, and described in detail hereinafter; while the tone units 353, 60|, 358, etc., are identical to the tone unit |600 illustrated in Fig. 16 and described in detail hereina-iter.

In the automatic exchange 50 the automatic switch trail |300 is of the International Standard Electric 7-A type, as previously noted; the manual dial toll board is of the well known Western Electric Company No. 3 type as previously noted, cord circuit |3|0 and the position circuit |3|6 being of the forms respectively in Figs. and 21 of the previously mentioned Wicks patent. Preferably, the toll register senders |39|, etc., and the local register senders |392, etc., are of the usual types; and the toll trunk circuits |303, etc., the CLR trunk circuits |302, etc., and the toll trunk circuits |3|1, etc., may be of any suitable types. Further, the tone units 10|, etc. are identical to the tone unit |600, illustrated in Fig. 16 and described in detail hereinafter. Preferably, the repeater circuits disposed in the automatic exchange 50 are identical to the repeater circuit 100, illustrated in Figs. 7 to 13, inclusive, and described in detail hereinafter. Preferably, the preselector |403 is of the form illustrated in Fig. 15 of the previously mentioned Wicks patent; and the coders |404, etc., are identical and, as indicated in Fig. 14, are substantially of the form illustrated in Fig. 16 (parts l, 2 and 3) of the previously mentioned Wicks patent. Preferably, the iinder circuits etc., are identical and are of the form illustrated in Fig. 17 of the previously mentioned Wicks patent; the register senders |402, etc., are identical and are of the form illustrated in Fig. 18 (parts and 2) of the previously mentioned Wicks patent; and the allotter |405 is of the form illustrated in Fig. 17 of the previously mentioned Wicks patent.

The repeater circuit 400 disposed in the automatic exchange 30 comprises, as shown in Figs. 4 to 6, inclusive, a relay group including a series relay R4H), a release relay R420, a line relay R430, two release pulse timer relays R435 and R440, a selector seize relay R450, an incoming release relay R460, a tandem test relay R410, a line supervisory relay R480, an answer supervisory relay R490, a re-ring relay R400, an answer supervisory relay R510, a supervisory control relay R520, a supervisory relay R530, a ring relay R535, three ring control relays R540, R550 and R560, a switch-over relay R510, a dial-in relay R580, a control relay R590, a loop test relay R500, a control relay R505, a supervisory relay R6|0, a signal-in relay R620, an interpulse timer relay R630, a sv/itch-througlfi relay R640, a guard relay R650, a supervisory pulse relay R660, a prepare relay R610, a send relay R615, a sequence relay R680, a pad control relay R690 and a pad cutout relay R600, as well as a toll pad 605.

The repeater circuit 100 as disposed in the automatic exchange comprises, as shown in Figs. 7 to 13, inclusive, a relay group including a pad cutout relay R1|0, a pad control relay R120, a send relay R130, a switch-through relay R140, a prepare relay R150, a cutoi relay R160, a time pulse relay R110, an auxiliary relay R180,

a re-ring relay R8 0, a control relay R820, an endof-dial relay R830, a supervisory relay R840, a supervisory start relay R650, a loop test relay R860, a signal-in relay R810, a dial-in relay R880, a selector seize relay R890, a switch-over relay R0l0, an incoming release relay R920, a control relay R330, a coder seizing relay R040, a control relay R950, a ring relay R960, a sequence relay R910, a finder stop relay R930, a monitor relay R900, a release pulse timer relay RI 0 0, an answer relay R1020, a release pulse timer relay R|030, a supervisory relay R|040, a series relay R|050, a line relay Rlil, a sleeve relay R|010, a start relay RI080, two transfer relays RI |10 and RI |20, a coder supervisory relay Ri |30, a register sender supervisory relay Rl |40, a control relay R| |50, a flash relay Ri i60, two re-ring relays Ril'ill and R22 i0, a sleeve relay R|220, a cutoff relay RIZSQ, a signal relay R1240, a position transfer relay R|250 and two switch release relays R|26 and R|210, as well as a toll pad 105.

As previously noted, the coder |404 disposed in the automatic exchange 50, shown in skeleton diagram in Fig. 14, is substantially of the form illustrated in Fig. 16 (parts l, 2 and 3) of the previously mentioned Wicks patent; however, the coder |404 is slightly mcdied in that it includes an additional relay, the transfer control relay Ri430, which is utilized in carrying out an additional feature incorporated in the coder |404 and explained more fully hereinafter. Otherwise, the coder is of theJ form mentioned and comprises the six identical digit storage devices DSI to DSS, inclusive; the digit storage device DS1 being illustrated as including the four individual relays A, B, C, and D.

The tone unit |600 isy of the type disclosed in the previously mentioned copending application of Otho D. Grandstai, and essentially comprises, as illustrated in Fig. 16, a receiving transformer TRI, a sending and coupling transformer TR2, a monitoring transformer TR3, a two-way amplifier Ti in the form of a vacuum tube of the well known pentode 606 type, a differential detector T2 in the form of a vacuum tube of the well known dual diode 65H6 type, an impulse amplier T3 in the form of a vacuum tube of the Well known pentcde 6C6 type and a signal amplifier T4 in the form of a vacuum tube of the well known pentode 43 type, as well as a relay group including a modulation cutoff relay R|620, a receiving switch relay Rill, a pulse cut-in relay R|650, an alarm relay R1655, an impulse-in relay R|660, and a signal-in relay R|610; together with a plate impedance |628 associated with the tube Tl, a plate impedance i634 associated with the tube T3, and a plate impedance |639 associated with the tube T4. Further, the tone unit |600 comprises circuit elements described in detail hereinafter and inter-connecting the elements mentioned, as well as a group of conductors extending to the associated repeater circuit, and including a sending conductor C||, a sending +conductor C|602, a monitoring -conductor C1603, a monitoring -|conductor C1604, a receiving conductor C|605, a receiving +Cenductor C 606, a receiving switch conductor C|601, a modulation cutoii conductor C|608, a signal-in conductor C|609, a dial-in conductor C|6I0, and a pulse cut-in conductor C|6| together with a 60 cycle conductor C|6|2 extending to an associated 60 cycle generator, not shown, and a 1000 cycle conductor C|6|3 extending to an associated 1000 cycle generator, not shown.

The essential details of a Strowger dial toll board and a cooperating Strowger switch train are disclosed in U. S. Patent No. 2,167,710, granted on August 1, 1939, to Thomas F. Crocker; while the essential details of a Western Electric Company No. 3 manual dial toll board and a cooperating International Standard Electric 7-A switch train are disclosed in U. S. Patent No. 1,125,579, granted on January 19, 1915, to Frank R. McBerty.

The operation of the tone unit 1600 In the tone unit |600, the tubes TI and T3 are each provided with a cathode heater having a nominal rating of 6.3 volts, at which voltage 0.3 ampere is drawn; the tube T2 is provided with a two-section cathode heater having a nominal rating of 6.3 volts, at which voltage 0.3 ampere is drawn; and the tube T4 is provided with a cathode heater having a nominal rating of 25 volts, at which voltage 0.3 ampere is drawn. The cathode heaters of the tubes TI, T2, T3, and T4 are connected in series circuit with a 4 ohm resistor RI and the winding of the alarm relay RI655 across a d8 volt exchange battery, the positive pole of which is grounded. More particularly the circuit mentioned extends from ground by way of the contacts |6|9 of a normally operated key K2, the cathode heater of the tube T2, a conductor Cl63, the cathode heater of the tube T3, a conductor CIEM, the cathode heater :u

of the tube TI, a conductor C|6'|5, the resistor RI, a conductor CI6T6, the cathode heater of the tube T4, and the winding of the alarm relay RI655 to the negative pole of the 48 volt exchange battery. Accordingly, the series circuit above traced is normally completed, whereby the cathode heaters heat the associated cathodes of the tubes mentioned rendering them electron emissive and eiecting operation of the alarm relay R|655. Of course, it will be understood that in the event any one of the tubes mentioned is removed from its socket or develops a burned out or broken cathode heater, the above traced series circuit is interrupted, causing the alarm relay RI655 to restore in order to complete, at the contacts |656, a circuit including the contacts |6I8 of the key K2 for illuminating a cathode heater alarm lamp LI65I, thereby to indicate the failure mentioned. Moreover, the potential drop along the above-traced series cathode heater circuit is employed at various points for grid biasing purposes, explained more fully hereinafter; and, in passing, it is noted that a negative potential of volts appears upon the conductor CI6'I3, a negative potential of 18.9 volts appears upon the conductor CIS'I5, and a negative potential of 20.1 volts appears upon the conductor Cl6'l6.

The tube TI is employed as a two-way amplifier, being utilized as an outgoing amplifier and modulator in the transmission of signals to the associated repeater circuit. The screen grid of the tube TI is supplied with positive potential from a 130 volt source of plate current, the negative pole of which is grounded, through an associated 50,000 ohm resistor I 629 and is by-passed to the cathode of the tube TI by a 0.1 mf. condenser !630 connected therebetween; and the suppressor grid of the tube Tl, employed as a modulation grid, is connected either to ground potential or to the 60 cycle conductor C|6| 2, depending unon the operated or restored condition of vthe modulation cutoff relay RI 620 and the receiving switch relay Rliidfl as explained more fully hereinafter.

For the reception of signals, the primary winding of the receiving transformer TRI is connected to the receiving conductor C|605 and to the receiving -I-conductor C|606 through the two illustrated 0.1 mf. condensers; the secondarywinding of the receiving transformer TRI is normally connected by Vway of the contacts I64I' Aof the normally operated receiving switch relay RI640 to the control grid of the tube TI in order to control the potential applied thereto; a negative potential of 20.1 volts being normally applied to the control grid of the tube TI from the conductor C|616 by way of a 0.15 megohm resistor R2, the secondary winding of the receiving transformer TRI and the contacts I64I of the receiving switch relay RIM-0. The cathode of the tube TI is normally connected by way of the contacts |643 of the normally operated receiving switch relay RI 640 to the conductor CI615 having negative 18.9 volt potential applied thereto. Ac-

. cordingly, the control grid of the tube TI has a negative biasing potential of 1.2 volts applied thereto with respect to the cathode by virtue of the 1.2 volt drop across the resistor RI.

With the receiving switch relay R|640 in its normal operated position, the suppressor grid of the tube TI is connected by way of the contacts |646 thereof to ground potential, as previously noted; and thesecondary winding of the sending and coupling transformer TR2 is connected, at the contacts |642 and |644 of the operated receiving switch relay R|640, across `the leads C|625 and C| 626. The primary winding of the monitoring transformer FR3 is connected by way of the two 5000 ohm resistors |623 and |620 across the leads C|625 and CI626; while the secondary winding of the monitoring transformer TRS is connected directly to the monitoring conductor C|603 and the monitoring -I-conductor C |664 extending to the associated repeater circuit. Thus, audible signals amplified by the tube TI and impressed through the sending and coupling transformer TR2 across the leads C|625 and C|-626` are supplied through the monitoring transformer TR3 to the monitoring and monitoring -i-conductors C|603 and C|604 extending to the associated repeater circuit, for a purpose more fully explained hereinafter. Further, the plate of the tube TI is connected by way of the associated 6500 ohm plate impedance |628 to the positive terminal of the 130 volt source of plate current supply, and is also connected by way of a 0.04 mf. coupling condenser |621 to one terminal of the primary winding of the sending and coupling transformer TR2, the other terminal of the `primary winding mentioned being grounded.

An inductance coil L3 is connected Aacross the leads C|625 and C|626 in series with a 0.04 mf. tuning condenser |63I, which condenser is shunted by the indicated trimmer condenser of such capacity as to render the couple seriesresonant at 1000 cycles. Finally, the junction between the inductance coil L3 and the tuning condenser |63I is connected to the right-hand plate of the tube T2, whereby a relatively high potential is placed on the right-hand plate of the tube T2 by the voltage drop across the inductance coil L3 when a 1000 cycle current is being received and amplified bythe tube Tl. In order to enable the system to respond `to impulses of '100 cycles plus or minus an amount equal to about 60 cycles, so as to allow for the reception of 60 cycle modulated, 1000 cycle'pulses, and to cut oil sharply when frequencies higher or lower than the allowable band are received, 

